1. Field of the Invention
The present invention relates to an infrared optical system, used for infrared cameras, for forming images of an object.
2. Description of the Related Art
Generally, infrared optical systems used in infrared cameras require the ability to form an image of an object on a detector. However, if the aberration of the infrared optical system is large, the object image formed by the camera becomes blurred, resulting in great decline in image quality. Therefore, infrared optical systems for the infrared cameras must have little aberration.
One method of realizing an infrared optical system exhibiting small aberration is to increase the number of lens elements as components. That is, the degree of freedom in terms of lens surface, lens thickness, lens-to-lens spacing, etc. is raised by increasing the number of lenses, to thereby enhance the aberration correcting capability of the infrared optical system. However, by increasing the number of lenses, the costs for the infrared optical system such as for lens material, fabrication and assembly also increase. Further, in addition to the rise in costs, there are also such problems as increased weight, expanded dimensions and more complicated adjustment of the assembly. It is therefore desirable in this respect to reduce the number of lenses.
FIG. 14 is a sectional view of a prior art infrared optical system shown in, e.g., [Design Example for the Use of Hybrid Optical Elements in the Infrared] by Max reported on pp.6833-6834, [Applied Optics Magazine] issued on Dec. 1, 1996.
This infrared optical system is used in an infrared-ray region having a wavelength band of 8-12 xcexcm, wherein extremely small aberration is attained by two pieces of germanium lens elements. In the following discussion, the object side (left side in the drawings) is referred to as the front surface of the lens, while the image side (right side in the drawings) is called the rear surface of the lens.
Referring to FIG. 14, a holding member 1 holds an aspherical diffraction lens 2 and a spherical lens 3 spaced away from each other. The aspherical diffraction lens 2 has a front surface 2a with a spherical shape and a rear surface 2b having a special configuration with a diffraction surface provided on an aspherical surface. The spherical lens 3 disposed on the image side from the aspherical diffraction lens 2 has a front surface 3a and a rear surface 3b, both of which are formed in a spherical shape. An object image obtained through the aspherical diffraction lens 2 and the spherical lens 3, is converted into electric signals by a detector 4 disposed facing the rear surface 3b of the spherical lens 3. The electric signals output from the detector 4 are converted into image signals by a signal processor (unillustrated), and the image signals are displayed as an infrared-ray image on a display unit (not shown).
In the thus constructed infrared optical system, the degree of freedom of the system is enhanced not by increasing the number of lens elements but by two kinds of effects imparted by the aspherical surface and the diffraction surface of the rear surface 2b of the aspherical diffraction lens 2 used therein. This degree of freedom is utilized to correct aberrations in the infrared optical system, and aberrations in the infrared optical system is thereby decreased. This reduces decline in image quality.
The infrared optical system described above involves the use of the diffraction surface for correcting the aberration. This diffraction surface, however, functions to deflect to optical path by utilizing the wave nature of light. That is, waves always propagate forward in a direction of wave surface alignment, and hence wave propagation direction is controlled by controlling the wave surface. In this case, as a matter of course, it is required that the wave surface be controlled at an accuracy smaller than the wavelength. Therefore, formation of the diffraction surface entails surface working having an accuracy smaller than the wavelength of the light. In the infrared optical system used for cameras for imaging the infrared ray having a wavelength on the order of, e.g., 10 xcexcm, hyper fine surface working having an accuracy smaller than at least 10 xcexcm is needed. Thus, a high-level working technique is necessary for forming the diffraction surface, and therefore costs rise and mass-production is hard to attain.
It is a primary object of the present invention, which was devised to obviate the above problems, to provide an infrared optical system for infrared cameras that is capable of reducing aberration without increasing the number of lens elements and without using hyperfine working.
To accomplish the above object, according to one aspect of the present invention, an infrared optical system for infrared cameras comprises a holding member, a convex lens held by the holding member and composed of a low-dispersion material transmitting infrared light, a stop, provided on an object-side on the basis of the convex lens, for restricting light beams entering the convex lens, and an aberration correcting plate, disposed in the vicinity of the stop and composed of a material transmitting the infrared light, for reducing spherical aberrations.